Transfer case for hybrid vehicle

ABSTRACT

A hybrid drive system for a four-wheel drive system arranged to supply motive power to a primary output shaft of a transfer case from an internal combustion engine and to a secondary output shaft from an electric motor/generator. The transfer case is comprised of a planetary gearset having an input driven by the motor/generator and an output directing drive torque to the secondary drivelines. This hybrid drive arrangement permits use of a modified transfer case in place of a conventional transfer case in a traditional four-wheel drive driveline.

FIELD OF THE INVENTION

The present invention relates generally to hybrid drive systems formotor vehicles and, more specifically, to a transfer case for use infour-wheel drive hybrid vehicles.

BACKGROUND OF THE INVENTION

Automobile manufacturers are actively working to develop alternativepowertrain systems in an effort to reduce the level of pollutantsexhausted into the air by conventional powertrains equipped withinternal combustion engines. Significant development has been directedto electric vehicles and fuel cell vehicles. Unfortunately, thesealternative powertrain systems suffer from several disadvantages and,for all practical purposes, are still under development. However,several different hybrid electric vehicles (HEV) have recently beenoffered for sale. These hybrid vehicles are equipped with an internalcombustion engine and an electric motor that can be operatedindependently or in combination to drive the vehicle.

There are two types of hybrid vehicles, namely, series hybrid andparallel hybrid. In a series hybrid vehicle, power is delivered to thewheels by the electric motor which draws electrical energy from thebattery. The engine is used in series hybrid vehicles to drive agenerator which supplies power directly to the electric motor or chargesthe battery when the state of charge falls below a predetermined value.In parallel hybrid vehicles, the electric motor and the engine can beoperated independently or in combination pursuant to the runningconditions of the vehicle. Typically, the control strategy for suchparallel hybrid vehicles utilizes a low-load mode where only theelectric motor is used to drive the vehicle, a high-load mode where onlythe engine is used to drive the vehicle, and an intermediate assist modewhere the engine and electric motor are both used to drive the vehicle.Regardless of the type of hybrid drive system used, hybrid vehicles arehighly modified versions of conventional vehicles that are expensive dueto the componentry, required control systems, and specialized packagingrequirements.

Hybrid vehicles have also been adapted to four-wheel drive vehicles andtypically utilize the above-noted parallel hybrid powertrain to drivethe primary wheels and a second electric motor to drive the secondarywheels. Obviously, such a four-wheel drive system is extremely expensiveand difficult to package. Thus, a need exists to develop hybridpowertrains for use in four-wheel drive vehicles that utilize manyconventional powertain components so as to minimize specializedpackaging and reduce cost.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a hybridpowertrain or drive system for a four-wheel drive vehicle.

In accordance with another object, the four-wheel drive hybrid drivesystem of the present invention includes a transfer case adapted forconnection between the transmission and the front and rear drivelines ofthe motor vehicle.

According to another object, the four-wheel drive hybrid drive system isa parallel-type system with a gearbox and an electric motor/generatorintegrated into the transfer case.

As a related object, the hybrid drive system of the present inventionpermits use of the internal combustion engine and the electricmotor/generator separately or in combination as power sources fordriving the motor vehicle.

These and other objects are provided by a transfer case adapted for usein a hybrid vehicle drive system having a primary power source, aprimary driveline, and a secondary driveline. The transfer case includesa primary output shaft connecting the primary power output shaftconnecting the primary power source to the primary driveline, asecondary output shaft connected to the secondary driveline, a gearboxhaving an input member and an output member connected to the secondaryoutput shaft, and a secondary power source connected to the input memberof the gearbox. The primary power source is a powertrain comprised of aninternal combustion engine and a transmission. The secondary powersource is an electric motor/generator.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating preferred embodiments of the present invention, are intendedfor purposes of illustration only since various changes andmodifications within the scope of this particular invention will becomeapparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a hybrid drive system for a four-wheel drive vehicle inaccordance with the present invention;

FIG. 2 is a schematic view of a transfer case associated with the hybridpowertrain of FIG. 1;

FIG. 3 is a schematic of an alternative construction for a transfer caseadapted for use in the hybrid drive system of FIG. 1; and

FIG. 4 is a diagram showing the control system associated with thehybrid drive systems of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, a four-wheel drive powertrain for a hybridmotor vehicle is shown to include a primary power source 10 comprised ofan internal combustion engine 12 and a transmission 14, a primarydriveline 16, a secondary driveline 18, and a secondary power source 20comprised of an electric motor/generator 22. The vehicle furtherincludes a powertrain control system 24 generally shown to include abattery 26, a group of vehicle sensors 28, and a controller 30. Primarydriveline 16 includes a first pair of wheels 32 connected to a firstaxle assembly 34 having a differential unit 36 connected to one end of aprimary prop shaft 38, the opposite end of which is connected to aprimary output shaft 40 of a transfer case 42. Similarly, secondarydriveline 18 includes a second pair of wheels 44 connected to a secondaxle assembly 46 having a differential unit 48 connected to one end of asecondary prop shaft 50, the opposite end of which is connected to asecondary output shaft 52 of transfer case 42.

In the embodiment shown, primary driveline 16 is the rear driveline suchthat motive power (i.e., drive torque) generated by engine 12 andtransmission 14 is transmitted to rear wheels 32 for establishing atwo-wheel/rear drive mode of operation. As will be detailed, electricmotor/generator 22 is operable to drive the secondary driveline that is,the front driveline, such that motive power is transmitted to frontwheels 44 for establishing either of a two-wheel/front drive mode or afour-wheel drive mode. It is to be understood that the present inventionis likewise applicable to a drivetrain arrangement in which primarypower source 10 transmits power to the front driveline while secondarypower source 20 transmits power to the rear driveline.

Referring now primarily to FIG. 2, transfer case 42 is shownschematically to include a housing assembly 56 of the type normallyadapted for bolted mounting to the casing of transmission 14. Primaryoutput shaft 40 is rotatably supported in housing 56 by bearingassemblies and is adapted for direct connection between the transmissionoutput shaft and primary prop shaft 38. In this manner, drive torquefrom engine 12 is transferred through transmission 14 and output shaft40 to primary driveline 16. Transfer case 42 further includes aplanetary gearset 58 operably installed between secondary output shaft52 and electric motor/generator 22. Gearset 58 includes ring gear 60that is non-rotationally fixed (i.e., such as to the housing a transfercase 42), a sun gear 62, and planet gears 64 meshed with sun gear 63 andring gear 60. Planet gears 64 are rotatably supported on pins 66 thatare fixed to a planet carrier 68. As seen, planet carrier 68 is fixedfor rotation with secondary output shaft 52 while sun gear 62 is fixedfor rotation with a rotor shaft 70 of motor/generator 22. Thus,energization of a stator 72 of motor/generator 22 causes driven rotationof rotor shaft 70 and sun gear 62 which results in rotation of planetcarrier 68 at a reduced speed, such that planetary gearset 58 acts as areduction gearset. While not intended to be limiting, it is contemplatedthat a preferred reduction ratio of about 3 to 1 is established by rearplanetary gearset 58.

Transfer case 42 is, in essence, not a transfer unit but rather a unitfor housing secondary power source 20 within the packagingconventionally provided for the transfer case in a four-wheel drivevehicle. A hybrid transfer case 142 is shown in FIG. 3 which isgenerally similar to transfer case 42 except that a selectivelyengageable clutch and shift system 80 is incorporated therein. System 80includes a transfer assembly 82 for selectively transferring drivetorque from primary output shaft 40 to secondary output shaft 52 toestablish a locked four-wheel drive mode. Transfer assembly 82 includesa first sprocket 84 rotatably supported on primary output shaft 40, asecond sprocket 86 fixed to secondary output shaft 52 and a power chain88 meshed therebetween. A mode clutch 90 is provided for selectivelycoupling first sprocket 84 to primary output shaft 40. Mode clutch 90 isshown as a synchronized dog clutch having a power-operated shiftactuator 92 operable for moving a dog sleeve 94 between a released andan engaged position. It will be understood that dog clutch can bereadily replaced with a power-operated) electromagnetic, hydraulic,pneumatic, motor-driven, etc.) multi-plate friction clutch betweenoutput shaft 40 and sprocket 84 such that the torque transfer can beadaptively controlled anywhere in a range between 100:0 to 50:50rear-to-front ratio.

The hybrid drive system of the present invention includes two drivepower sources, namely internal combustion engine 12 and motor/generator22. Power from engine 12 is transmitted to transmission 14 which, inturn, is delivered to transfer case 42 via the transmission outputshaft. Transmission 14 can be of any known type (i.e., automatic,manual, automated manual, CVT) having a forward-reverse switchingmechanism and a gearshift mechanism. Motor/generator 22 is connected tobattery 26 and can be selectively placed in any of a DRIVE state, aCHARGING state, and a NO-LOAD state by controller 30. In the DRIVEstate, motor/generator 22 functions as an electric motor which is drivenby electric energy supplied from battery 26. In the CHARGING state,motor/generator 22 functions as an electric generator with regenerativebraking (brake torque electrically generated by motor/generator 22) forstoring electrical energy in battery 26. In the NO-LOAD state, theoutput (i.e., rotor 70) of motor/generator 22 is permitted to rotatefreely.

As noted, control system 24 is provided for controlling operation of thehybrid powertrain shown in FIG. 1 equipped with either of transfer cases42 or 142. Referring to FIG. 4, controller 30 is shown to receive inputsignals from various sensors and input devices previously identifiedcumulatively in FIG. 1 as vehicle sensors 28. Controller 30 isprincipally comprised of a microcomputer having a central processingunit (CPU), random-access memory (RAM), read-only memory (ROM), and aninput-output actuator interface. Controller 30 performs data processingoperations to execute various control routines according to controlprograms and/or maps stored in the ROM. Controller 30 receives data froman ignition switch 130, a gearshift lever switch 132, an acceleratorposition sensor 134, a brake status switch 136, a battery temperaturesensor 138, a battery SOC (state of charge) sensor 140, and a throttleposition sensor 142. In addition, other inputs include an engine speedsensor 144, a motor speed sensor 146, a rear shaft speed sensor 148, anda front shaft speed sensor 150. Ignition switch 130 is closed when thevehicle key is turned on. Assuming transmission 14 is of an automatictype, then “P”, “N”, “R”, and “D” switches in gearshift selector switch132 are closed when the gearshift mechanism is located in its Park (P),Neutral (N), Reverse (R) and Drive (D) positions, respectively.Accelerator position sensor 134 senses the depression angle of anaccelerator pedal. Brake status switch 136 is turned on when the brakepedal is depressed. Battery temperature sensor 138 senses thetemperature of battery 26. Battery SOC sensor 140 senses the chargelevel of battery 26. Throttle position sensor 142 senses the degree ofopening of the engine throttle valve. Engine speed sensor 144 senses aparameter indicative of the rotary speed of the drive shaft of engine12. Motor speed sensor 146 senses a parameter indicative of the rotaryspeed of rotor 70 of motor/generator 22. Rear speed sensor 148 sensesthe rotary speed of either rear output shaft 40 or rear propshaft 38 andcan further be used as an indication of vehicle speed. Front speedsensor 150 senses the rotary speed of either front output shaft 52 orfront prop shaft 50.

Based upon the operating information inputted to controller 30, a modeof operation of the hybrid powertrain is selected and controller 30sends electric control signals to the various power-operated controlleddevices. Specifically, controller 30 monitors and continuously controlsactuation of motor/generator 22. Additionally, controller 30 monitorsand controls various engine management systems for controlling the speedand torque generated by engine 12. These include a fuel injection system152, an ignition timing system 154, and a valve timing system 156. A lowvoltage auxiliary battery 158 may serve as the power supply forcontroller 30.

There are four modes of operation for vehicle 10, namely: (a) anelectric mode; (b) a hybrid; (c) an engine mode; and (d) a regenerativemode. In the electric mode, only motor 22 provides motive power tovehicle 10. In the hybrid mode, both engine 12 and motor 22 providemotive power to vehicle 10. In the engine mode, only engine 12 providesmotive power to vehicle 10. In the regenerative mode, a portion of theengine power is absorbed by motor/generator 22 to charge battery 26. Thetransition from one mode to the next is smooth and transparent to thevehicle operator since controller 30 selects the most appropriate modedepending on various vehicle operating conditions including vehiclespeed, accelerator demand and battery charge status.

During operation in the electric mode, it is contemplated that thevehicle may be driven up to about thirty miles per hour. This permitsuse of the electric mode in urban areas and stop and go driving so as toreduce the overall fuel consumption of vehicle 10.

When shifting from the electric mode into the hybrid mode, engine 12 isstarted. Such that engine 12 and transmission 14 deliver power toprimary driveline 16 while motor/generator 22 delivers power tosecondary driveline 18. Controller 30 controls engine torque viareal-time control of the various engine management systems inconjunction with controlling the motor torque developed bymotor/generator 22. Under light throttle conditions, motor/generator 22may be placed in its CHARGING state to recharge battery 26.

When operating conditions of vehicle 10 warrant operation in the engineonly mode, the hybrid drive system is switched by simply shiftingmotor/generator 22 into its NO-LOAD state. Additionally, motor/generator22 can be shifted into its CHARGING state to provide regenerativebraking. A mode selector 106 is provided for permitting the vehicleoperator to intentionally select operation in one of a AUTO mode and a4WD mode. Based on the particular drive mode selected, controller 30controls the actuated condition of mode clutch 90. As such, varioustwo-wheel drive and four-wheel drive modes are available at all times.Thus, vehicle 10 combines the commercially-successful features of atraditional four-wheel drive drivetrain architecture (engine,transmission and transfer case) with hybrid power control tosignificantly advance the hybrid drive technology. Moreover, the presentinvention provided an arrangement for a hybrid four-wheel drive vehiclewhich is not highly customized, but rather permits “drop-in” assembly ofa hybrid transfer case in place of a conventional transfer case. Thisarrangement also permits the use of a smaller internal combustion enginethat is sized for cruise operation while the electric assist of themotor/generator is capable of driving the vehicle at low speeds.

Preferred embodiments of the invention have been disclosed to providethose skilled in the art an understanding of the best mode currentlycontemplated for the operation and construction of the four-wheel drivehybrid drive system. The invention being thus described, it will beobvious that various modifications can be made without departing fromthe true spirit and scope of the invention, and all such modificationsas would be considered by those skilled in the art are intended to beincluded within the scope of the following claims.

What is claimed is:
 1. A transfer case for use in a motor vehicle havingan engine and first and second drivelines, comprising: a housing; afirst output shaft rotatably supported by said housing and adapted forconnection between the engine and the first driveline; a second outputshaft rotatably supported by said housing and adapted for connection tothe second driveline; a gearset having a sun gear, a ring gear fixed tosaid housing, and pinion gears meshed with said ring gear and said sungear, said pinion gears are rotatably supported on a pinion carrierwhich is fixed to said second output shaft; and an electric motor forselectively driving said sun gear.
 2. The transfer case of claim 1further comprising a control system for controlling actuation of saidelectric motor to define an electric operating mode, a hybrid operatingmode, and an engine operating mode.
 3. The transfer case of claim 2wherein said electric operating mode is established with said electricmotor actuated for driving said sun gear.
 4. The transfer case of claim2 wherein said hybrid operating mode is established with the enginedelivering power to the first driveline while said electric motordelivers power to the second driveline.
 5. The transfer case of claim 2wherein said control system includes a controller and sensors fordetecting operating characteristics of the vehicle and sending sensorinput signals to said controller, said controller operable to sendcontrol signals to said electric motor.
 6. The transfer case of claim 2further comprising a mode clutch for selectively coupling said firstoutput shaft to said second output shaft, said mode clutch having apower-operated clutch operator which is controlled by said controlsystem.
 7. The transfer case of claim 6 having a transfer unit includinga first sprocket rotatably supported on said first output shaft, asecond sprocket fixed to said second output shaft, a power chainconnecting said first and second sprockets, wherein said mode clutch isoperable in a released mode to permit rotation of said first sprocketrelative to said first output shaft and in an engaged mode to rotativelycouple said first output shaft to said first sprocket.
 8. The transfercase of claim 7 wherein said control system permits selection of atwo-wheel drive mode, a part-time four-wheel drive mode and an automaticfour-wheel drive mode, said two-wheel drive mode is established withsaid mode clutch in its released mode, said part-time four-wheel drivemode is established with said mode clutch in its engaged mode, and saidautomatic four-wheel drive mode is established with said mode clutchvaried between said engaged and released modes.
 9. A hybrid motorvehicle, comprising: a powertrain including an internal combustionengine and a transmission; a first driveline including a firstdifferential connecting a pair of first wheels; a second drivelineincluding a second differential connecting a pair of second wheels; atransfer case including a first output shaft driven by said powertrainand operably connected to said first differential, a second output shaftoperably connected to said second differential, a planetary gearsethaving a sun gear, a fixed ring gear, and pinion gears meshed with saidring gear and said sun gear, said pinion gears are rotatably supportedon a pinion carrier which is fixed to said second output shaft, and anelectric motor for selectively driving said sun gear; and a controlsystem for controlling actuation of said electric motor to define anelectric operating mode, a hybrid operating mode, and an engineoperating mode.
 10. The hybrid motor vehicle of claim 9 wherein saidelectric operating mode is established with said electric motor actuatedfor driving said sun gear.
 11. The hybrid motor vehicle of claim 9wherein said hybrid operating mode is established with said powertrainactuated for driving said first output shaft and said electric motoractuated for driving said second output shaft.
 12. The hybrid motorvehicle of claim 9 wherein said control system includes a controller andsensors for selecting operating characteristics of the vehicle andsending sensor input signals to said controller, said controlleroperable to send control signals to said electric motor.
 13. (previouslypresented) The transfer case of claim 9 further comprising a mode clutchfor selectively coupling said first output shaft to said second outputshaft, said mode clutch having a power-operated clutch operator which iscontrolled by said control system.
 14. The transfer case of claim 13having a transfer unit including a first sprocket rotatably supported onsaid first output shaft, a second sprocket fixed to said second outputshaft, a power chain connecting said first and second sprockets, whereinsaid mode clutch is controlled by said control system and is operable ina released mode to permit rotation of said first sprocket relative tosaid first output shaft and in an engaged mode to rotatively couple saidfirst output shaft to said first sprocket.
 15. The transfer case ofclaim 14 wherein said control system permits selection of a two-wheeldrive mode, a part-time four-wheel drive mode and an automaticfour-wheel drive mode, said two-wheel drive mode is established withsaid mode clutch in its released mode, said part-time four-wheel drivemode is established with said mode clutch in its engaged mode, and saidautomatic four-wheel drive mode is established with said mode clutchvaried between said engaged and released modes.
 16. A hybrid motorvehicle, comprising: a powertrain including an internal combustionengine and a transmission; a first driveline including a firstdifferential connecting a pair of first wheels; a second drivelineincluding a second differential connecting a pair of second wheels; atransfer case including a first output shaft driven by said powertrainand operably connected to said first differential, a second output shaftoperably connected to said second differential, a planetary gearsethaving a sun gear, a fixed ring gear, and pinion gears meshed with saidring gear and said sun gear, said pinion gears are rotatably supportedon a pinion carrier which is fixed to said second output shaft, atransfer unit coupled to said second output shaft, a mode clutchoperable in an engaged mode to couple said transfer unit to said firstoutput shaft and in a released mode to disengage said transfer unit fromcoupled engagement with said first output shaft, and an electric motorfor driving said sun gear; and a control system for controllingactuation of said mode clutch and said electric motor to define anelectric operating mode, a hybrid operating mode, and an engineoperating mode.
 17. A hybrid vehicle, comprising: an internal combustionengine; a transmission driven by said engine; a front drivelineincluding a pair of front wheels, a front axle assembly driving saidfront wheels, and a front propshaft driving said front axle assembly; arear driveline including a pair of rear wheels, a rear axle assemblydriving said rear wheels, and a rear propshaft driving said rear axleassembly; a transfer case having a rear output shaft driven by saidtransmission and driving said rear propshaft, a front output shaftconnected to said front propshaft, a planetary reduction gearset havingan input member and an output member driving said front output shaft,and an electric motor driving said input member; sensors for detectingoperational characteristics of the vehicle; and a controller operable toreceive sensor signals from said sensors and control actuation of saidelectric motor, wherein an electric two-wheel drive mode is establishedwhen said electric motor drives said front output shaft to deliver drivetorque to said front wheels while said engine is shut-off such that nodrive torque is delivered to said rear wheels.
 18. The hybrid vehicle ofclaim 17 wherein an engine two-wheel drive mode is established when saidelectric motor is shut-off and said engine delivers power to saidtransmission for driving said rear driveline through said rear outputshaft.
 19. The hybrid vehicle of claim 17 wherein a four-wheel drivemode is established when said engine delivers drive torque to said reardriveline and said motor delivers drive torque to said front driveline.20. The hybrid vehicle of claim 17 wherein said electric motor can beused to recharge a battery.
 21. A transfer case for use in a motorvehicle having an engine and first and second drivelines, comprising: ahousing; a first output shaft rotatably supported by said housing andadapted for connection between the engine and the first driveline; asecond output shaft rotatably supported by said housing and adapted forconnection to the second driveline; a gearset having a sun gear, a ringgear fixed to said housing, and a carrier fixed for rotation with saidsecond output shaft and rotatably supporting pinion gears meshed withsaid sun gear and said ring gear; an electric motor for selectivelydriving said sun gear; and a mode clutch for selectively coupling saidfirst output shaft to said second output shaft.
 22. The transfer case ofclaim 21 further comprising a control system including a controller andsensors for detecting operating characteristics of the vehicle andsending input signals to said controller, said controller operable tosend control signals to said electric motor.
 23. The transfer case ofclaim 22 wherein said mode clutch includes a power-operated actuatorwhich is controlled by said controller.
 24. The transfer case of claim23 wherein said control system permits selection of a two-wheel drivemode, a part-time four-wheel drive mode and an automatic four-wheeldrive mode, said two-wheel drive mode is established with said modeclutch in its released mode, said part-time four-wheel drive mode isestablished with said mode clutch in its engaged mode, and saidautomatic four-wheel drive mode is established with said mode clutchvaried between its engaged and released modes.
 25. The transfer case ofclaim 21 having a transfer unit including a first sprocket rotatablysupported on said first output shaft, a second sprocket fixed to saidsecond output shaft, a power chain connecting said first and secondsprockets, wherein said mode clutch is operable in a released mode topermit rotation of said first sprocket relative to said first outputshaft and in an engaged mode to rotatively couple said first outputshaft to said first sprocket.
 26. A hybrid motor vehicle, comprising: apowertrain including an internal combustion engine and a transmission; afirst driveline including a first differential connecting a pair offirst wheels; a second driveline including a second differentialconnecting a pair of second wheels; a transfer case including a firstoutput shaft driven by said powertrain and operably connected to saidfirst differential, a second output shaft operably connected to saidsecond differential, a mode clutch for selectively coupling said firstoutput shaft to said second output shaft, a planetary gearset having asun gear, a fixed ring gear, and pinion gears meshed with said ring gearand said sun gear, said pinion gears are rotatably supported on a pinioncarrier which is fixed to said second output shaft, and an electricmotor for selectively driving said sun gear; and a control system forcontrolling actuation of said electric motor and said mode clutch. 27.The hybrid motor vehicle of claim 26 wherein an electric operating modeis established when said electric motor is actuated for driving said sungear.
 28. The hybrid motor vehicle of claim 26 wherein a hybridoperating mode is established when said powertrain is actuated fordriving said first output shaft and said electric motor is actuated fordriving said second output shaft.
 29. The hybrid motor vehicle of claim20 having a transfer unit including a first sprocket rotatably supportedon said first output shaft, a second sprocket fixed to said secondoutput shaft, a power chain connecting said first and second sprockets,wherein said mode clutch is operable in a released mode to permitrotation of said first sprocket relative to said first output shaft andin an engaged mode to rotatively couple said first output shaft to saidfirst sprocket.
 30. The hybrid motor vehicle of claim 29 wherein saidcontrol system permits selection of a two-wheel drive mode, a part-timefour-wheel drive mode and an automatic four-wheel drive mode, saidtwo-wheel drive mode is established with said mode clutch in itsreleased mode, said part-time four-wheel drive mode is established withsaid mode clutch in its engaged mode, and said automatic four-wheeldrive mode is established with said mode clutch varied between saidengaged and released modes.
 31. A hybrid motor vehicle, comprising: apowertrain including an internal combustion engine and a transmission; afirst driveline including a first differential connecting a pair offirst wheels; a second driveline including a second differentialconnecting a pair of second wheels; a transfer case including a firstoutput shaft driven by said powertrain and operably connected to saidfirst differential, a second output shaft operably connected to saidsecond differential, a planetary gearset having a sun gear, a fixed ringgear, and pinion gears meshed with said ring gear and said sun gear,said pinion gears are rotatably supported on a pinion carrier which isfixed to said second output shaft, a transfer unit coupled to saidsecond output shaft, a mode clutch operable in an engaged mode to couplesaid transfer unit to said first output shaft and in a released mode todisengage said transfer unit from coupled engagement with said firstoutput shaft, and an electric motor for driving said sun gear; and acontrol system for controlling actuation of said mode clutch and saidelectric motor to define an electric operating mode, a hybrid operatingmode, and an engine operating mode.